The type of motion detector here considered comprises a device for sensing incident radiation, usually infrared rays, which may be emitted by an associated source elsewhere in the space under surveillance and whose interruption by an intruder sets off an alarm. Alternatively, such a device can be used to sense heat waves from the body of the intruding person.
Usually, e.g. as known from U.S. Pat. No. 3,958,118, devices of this kind comprise a plurality of closely juxtaposed sensors toward which incident rays from various zones--referred to hereinafter as fields of view--of the surveyed space are directed by suitable focusing means. Thus, an intruder moving through that space will consecutively activate several such sensors and thereby give rise to output signals distinguishable from random noise or background radiation. In the system of the aforementioned U.S. Patent, the outputs of all the sensors are connected in parallel to the gate of a field-effect transistor working into an analog amplifier which differentiates the output signal of any sensor so as to generate pulses of opposite polarities of the leading and trailing edges of that signal. These pulses, upon integration and if above a certain threshold, are fed to an AND gate to trigger an alarm generator whenever the interval between the two opposite-polarity pulses is short enough and their magnitudes are large enough to let their integration products overlap. The threshold is so chosen that the motion detector responds only to at least two consecutive output pulses taken as an indication that an intruder has traversed two adjoining fields of view.
So-called window discriminators designed for the establishment of certain time periods, operating with fixed voltage thresholds, generally must include circuits with large time constants designed to prevent spurious triggering. These time constants, which may have magnitudes on the order of several minutes depending on the number of stages, tend to delay the activation or reactivation of a motion detector and may therefore unduly impede the work of a service person testing its operation. Moreover, minor irregularities such as manufacturing tolerances and capacitor leakages may have an unbalancing effect which may cause false alarms even in these cases.